A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser source for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as droplets of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
Another known method of producing EUV radiation is known as dual laser pulsing (DLP). In the DLP method droplets are pre-heated, for instance by a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser to cause the droplet (e.g., a tin droplet) to decompose into vapor and small particles that are then heated to a very high temperature by a CO2 laser.
In known methods such as LPP and DLP methods, a stream of droplets is generated. The droplets may be generated as either a continuous stream or in pulses.
For example, in one known method that is used in particular for LPP methods, a heated container is filled with molten tin that passes from the container to a capillary via a filter and a piezoelectric actuator. A continuous jet exits from the end of the capillary that is modulated in velocity by the piezoelectric actuator. During flight, this jet decomposes into small droplets and due to the modulated velocity, these smaller droplets merge into larger droplets spaced at larger distances.
The laser beam that pre-heats the droplet to cause the droplet to decompose into vapor and small particles may be slightly misaligned with respect to the droplet it pre-heats. Such slight misalignment may cause further misalignment when the CO2 laser heats the vapor and the small particles to the very high temperature. Such further misalignment may be detrimental to the amount of EUV radiation emitted by the resulting plasma.